The influence of moisture level, light, aeration, and glucose upon C2H2 reduction by a clay soil was studied. C2H2 reduction was greater in 15 g samples of the air-dried soil moistened with 9 ml water than in samples moistened with 5 ml water. Illumination of the soil led to significantly more C2H2 reduction than when the soil samples were incubated in the dark. The addition of glucose to the soil increased C2H2 reduction. C2H2 reduction by soil samples incubated under an aerobic gas phase (either Ar + O2, 80; 20; or air) was not significantly different from that of samples incubated under an anaerobic gas phase (Ar). Several questions regarding the use of the C2H2 reduction technique in assessing nitrogen fixation by natural ecosystems are raised.